1. Field of the Invention
The present general inventive concept relates to an optical pickup apparatus, and more particularly, to an optical pickup apparatus having a multi-sectional polarizer.
2. Description of the Related Art
An optical pickup apparatus is used to record information onto an optical disc using light or to reproduce the information recorded on the optical disc. The optical pickup apparatus includes an objective lens to focus light onto an optical disc. Representative examples of an optical disc may include a Compact Disc (CD) using a laser with a wavelength of 780 nm, and a Digital Versatile Disc (DVD) using a laser with a wavelength of 650 nm. Recently, in order to further increase the recording density, a Blu-ray Disc (BD) using a blue-violet laser with a wavelength of about 405 nm has been used.
A blu-ray disc uses light with a shorter wavelength compared to a CD and a DVD, and accordingly a numerical aperture (NA) of an objective lens for the blu-ray disc is greater than a numerical aperture of an objective lens for the CD and a numerical aperture of an objective lens for the DVD. For example, the objective lens for the CD has a numerical aperture of 0.45, the objective lens for the DVD has a numerical aperture of 0.55, and the objective lens for the blu-ray disc has a numerical aperture of 0.85.
Herein, the numerical aperture refers to a parameter defined as a combination of an external diameter and a focal distance of a lens. In general, as a numerical aperture of a lens increases, the lens becomes more convex. Accordingly, in order to minimize an optical aberration, a lens having a greater numerical aperture requires more precise processing.
In other words, the objective lens for the blu-ray disc having a numerical aperture significantly greater than the numerical aperture of the objective lens for the CD and numerical aperture of the objective lens for the DVD needs to be manufactured through processes with high precision. Accordingly, the objective lens for the blu-ray disc may have a narrow allowable range of manufacturing tolerances, and thus costs of an optical pickup apparatus for the blu-ray disc may increase.
An optical pickup apparatus may be compatible with a CD, a DVD and a blu-ray disc, so as to be employed for the use of the CD, the DVD and the blu-ray disc. This compatible optical pickup apparatus may include a compatible objective lens to be applied to all the CD, the DVD and the blu-ray disc.
To use the compatible optical pickup apparatus, it is particularly important that a working distance (WD) between the compatible objective lens and the CD should be ensured to be equal to or greater than a default value, for example 0.4 mm. If the working distance does not reach the default value, it is highly likely that inadvertent contact between the CD and the objective lens occurs during working, which results in damage to the CD or the objective lens.
In the case of using the blu-ray disc, when the compatible objective lens has a numerical aperture of 0.85, a focal distance is substantially equal to or greater than 2.2 mm so that the working distance between the CD and the compatible objective lens may be ensured to be equal to or greater than 0.4 mm.
However, the compatible objective lens having the focal distance of about 2.2 mm with respect to the blu-ray disc is contrary to a demand for slimness of the optical pickup apparatus, taking into consideration that an objective lens currently used in a slim optical pickup apparatus has a focal distance of about 1.4 mm with respect to the blu-ray disc.